Gas turbine engines, also known as combustion turbine engines are rotary engines that extract energy from a flow of combusted gases passing through the engine onto a multitude of turbine blades. Gas turbine engines have been used for land and nautical locomotion and power generation, but are most commonly used for aeronautical applications such as for airplanes and helicopters, where they primarily are used for propulsion.
Gas turbine engines usually power a number of different accessories such as generators, starter/generators, permanent magnet alternators (PMA) and/or permanent magnet generators (PMG), fuel pumps, and hydraulic pumps. All these accessories provide for functions needed on the aircraft other than propelling the aircraft. For example, when the gas turbine engine is running the starter generator (S/G) produces electrical power and when the gas turbine needs to be started the S/G serves as a starting motor when provided energy from another energy source.